(1) Technical Field
The present invention relates to the detection of eavesdropping devices connected to a local telephone line at the subscriber's terminal and more particularly, to devices of the line bridging type where a small change in line impedance occurs because of the bridging device.
(2) Background Art
Many types of eavesdropping devices have been developed for both legal and illegal monitoring of telephone conversations. Such devices include microphone pickups and transmitting devices ("bugs"), and radio frequency devices which pick up electromagnetic radiation from a subscriber's terminal. A different but very common type of eavesdropping equipment includes a simple line "tap" where a bridging amplifier is placed across the telephone line and the output thereof is utilized to intercept conversations on the telephone line.
Assuming that a subscriber's terminal is periodically inspected for "bugs" and that it is well shielded to prevent radio frequency emissions, the next most likely way for eavesdropping to occur is to utilize the line tapping technique. This tapping technique will cause a small or perhaps even a large change in line impedance depending upon how the tapping is accomplished.
Accordingly it is the object of this invention to show how a line tapping device can be detected on the line by the subscriber's telephone terminal equipped with suitable microprocessor controlled electronics.